The present invention relates to a memory device, and more particularly to a memory device including a magnetic tunnel junction device.
A dynamic random access memory (DRAM), which is one of the most widely used semiconductor memory device, exhibits high operation speed and high integration. However, the DRAM is a volatile memory device and loses data upon power-off, and during operation, a refresh process is performed on the memory to prevent loss of stored data. Meanwhile, a flash memory is a non-volatile memory device and can be manufactured in high integration. However, the flash memory has a slow operation speed. As an alternative for the DRAM and the flash memory, a magneto-resistance random memory device (MRAM) has been developed and exhibits non-volatility, high operation speed, and high integration (scalability) characteristics.
More specifically, an MRAM device is a non-volatile memory device where data is stored in magnetic storage elements having a different resistance according to magnetic field changed between ferromagnetic plates. The magnetic storage element is a component including two ferromagnetic plates separated by an insulating layer. If polarities of the two ferromagnetic plates are parallel (the same), resistance of the magnetic storage element is minimized. Otherwise, if polarities of the two ferromagnetic plates are opposite, the resistance is maximized. The MRAM device stores data based on a change in resistance of a cell according to magnetization of ferromagnetic plates in the magnetic storage element. As a magnetic storage element, a Magnetic Tunnel Junction (MTJ ELEMENT) is widely used.
In an MRAM, a MTJ element generally includes a stacked structure of a ferromagnetic layer, an insulating layer, and another ferromagnetic layer. When electrons passing through a first ferromagnetic layer penetrate into an insulating layer serving as a tunneling barrier, electron's probability to penetrate into the insulating layer is determined by magnetic direction of second ferromagnetic layer. If two ferromagnetic layers have the same polarity (parallel magnetic direction), amount of current tunneling the insulating layer is maximized. Otherwise, if two ferromagnetic layers have opposite magnetic directions, amount of current is minimized. For example, when resistance recognized based on the tunneling current is high, information stored in the MTJ element is a logic level “1” (or “0”). If the resistance is low, information is a logic level “0” (or “1”). Hereinafter, one of two ferromagnetic layers is called a pinned layer because its polarity is fixed to a certain polarity, but the other is called a free layer because its polarity can be changed according to an applied magnetic field or current.
Here, manufacturing an MRAM device may be difficult. For instance, a microstructure of a ferromagnetic layer and an insulating layer for a MRAM device may be difficult to fabricate. Further, semiconductor process machines may not be advanced enough to pattern layers of a ferromagnetic layer and an insulating layer.